1. Field of the Invention
The present invention generally relates to display devices and, more particularly, is directed to a display device having a beam index type (beam-indexing) cathode ray tube (CRT).
2. Description of the Prior Art
In general, the beam index type CRT has a fluorescent screen which is constituted by forming red, green and blue fluorescent materials in strip configurations and then by forming at a constant pitch material which generates secondary electrons or ultraviolet rays, that is, index fluorescent materials in strip configurations. The beam index type CRT selects a color to be illuminated in a manner that a position of an electron beam is detected by detecting secondary electrons or ultraviolet rays which are instantly generated from the fluorescent screen upon horizontal scanning of an electron beam and then the electron beam is controlled in accordance with the detected position.
The beam index type CRT has the following features since it does not need a color selection electrode such as a shadow mask for seizing a beam current:
(1) an electron beam is utilized at a high efficiency, and so luminance of the CRT is high and electric power consumption thereof is low; PA1 (2) the electron beam is a single beam, and so there is no problem of beam concentration; PA1 (3) the CRT is hardly influenced by earth magnetism; and PA1 (4) the construction of the tube is simple.
Recently, the beam index type CRT has been employed as a display device for a cockpit of an airplane, for example, due to the high luminance thereof. Namely, it is an absolute condition for the display device for the cockpit that image information can be surely recognized by a pilot in various conditions such as the day time where sunlight is directly incident on the display and the nighttime where there is no outdoor daylight. To this end, the display device is required to have a high luminance and a high contrast, so that the beam index type CRT has been employed as a display device for the cockpit due to the high luminance thereof.
However, the beam index type CRT has the disadvantage such that the screen is always illuminated since an electron beam is required to be always irradiated on the screen. The lowest luminance level of the screen due to this electron beam is called a black level or a black luminance level, in general. This black level does not cause any problem in the daytime about recognizing an image information on the screen, but causes an important problem in the nighttime. Namely, luminance of the background image of the screen due to the black level becomes brighter in the nighttime, so that it becomes difficult to recognize an image information.
In order to decrease the luminance of the black level, it is required to decrease an amount of the electron beam. However, if an amount of the electron beam is simply decreased, an amount of secondary electrons or ultraviolet rays generated from the fluorescent screen upon impingement of the electron beam thereon also decreases, so that the detection of the secondary electrons or ultraviolet rays becomes difficult.
In order to obviate this problem, the conventional display device employs, as an element for detecting the secondary electrons or ultraviolet rays, an element having a function of multiplying the detected beams or rays, for example, a photomultiplier. The photomultiplier is a photo sensor with a high sensitivity, a low noise and a high-speed responsibility having a function of multiplying secondary electrons.
FIG. 1 of the accompanying drawings shows an example of conventional display devices utilizing the photomultipliers. Referring to FIG. 1, this display device comprises a photomultiplier 12 which detects, through an optical filter 11, light which is generated from the beam index type CRT (hereinafter referred to as a CRT) upon impingement of the electron beam on the fluorescent screen thereof and then generates an electric signal (index signal) Si depending on an amount of the detected light.
The index signal Si from the photomultiplier 12 is amplified by an amplifier 13 and then supplied to an index signal processing circuit 14. The index signal processing circuit 14 changes over inputted red (R), green (G) and blue (B) signals in accordance with the index signal Si from the photomultiplier 12 through the amplifier 13 to thereby output the R, G and B signals alternately in time sequence to the CRT.
A luminance level of the CRT is determined by adjusting a luminance level control volume 15. That is, the index signal processing circuit 14 controls a gain of each of the R, G and B signals in accordance with a luminance level control signal Sv from the volume 15.
The photomultiplier 12 is supplied with a power supply voltage Vd so as to maintain a constant multiplication ratio thereof from a power supply circuit 16 to which an external constant voltage V is supplied.
However, the thus constituted conventional display device has the following disadvantages.
When the multiplication ratio of the photomultiplier 12 is set to be low, since a signal level of the index signal Si becomes lower, it may become difficult to detect the position of the electron beam, so that it is required to set an initial luminance level of the CRT higher. As a result, the black level becomes higher and so image quality is degraded, thereby recognition of image information becomes difficult.
Conversely, when the multiplication ratio of the photomultiplier 12 is set to be higher, a level of the index signal Si saturates in a high luminance mode, so that it becomes impossible to detect the position of the electron beam, thereby a normal image can not be reproduced on the screen.
The fluorescent screen of the CRT has an ineffective screen area and an effective screen area. In the effective screen area, red, green and blue fluorescent materials are formed separately and carbon materials are formed therebetween, and further index fluorescent materials are formed on the carbon materials with a predetermined distance. In the ineffective screen area, carbon is formed on the entire surface of this area and index fluorescent materials are formed on the carbon with a predetermined distance.
The scanning by an electron beam is started from an end portion of the ineffective screen area. In this case, when light generated from the first index fluorescent material on the ineffective screen area is not detected, the change over timing of the R, G and B signals shifts from the correct timing. As a result, for example, the red fluorescent material is not excited by the R signal but excited by the G or B signal, so that a normal image can not be reproduced on the screen.
When the ineffective screen area is scanned by an electron beam, only a detection signal (white level signal) from the index signal processing circuit 14 is supplied to the CRT, whereby the index fluorescent materials on the ineffective screen area are excited by the detection signal. In contrast, when the effective screen area is scanned by an electron beam, one of the R, G and B signals selectively changed over by the index processing circuit 14 and the detection signal are added and then supplied to the CRT.
In the thus constituted conventional display device, since a level of the detection signal is kept to a constant value, a signal level of the index signal Si obtained from the photomultiplier 12 with the fixed amplification ratio by scanning the first index fluorescent material changes largely, whereby it is difficult to surely detect the index signal corresponding to the first index fluorescent material, thereby a normal image can not be reproduced.